docs: repair AIXiamo sponsor guide links in four languages

[daichongaixiamo]

Summary

Replace the unavailable AIXiamo sponsor-guide GitHub URLs with the maintained self-hosted tutorial directory, https://gpt-guide.aixiamo.com/knowledge, in the Chinese, English, Russian, and Korean READMEs.

This is an update to an existing sponsor resource, not a new sponsor placement or an independent endorsement. The current guide directory covers Plus/Pro purchases, new subscriptions, renewals, account verification, and Codex-related questions.

Scope

• Exactly one resource href changed in each of README.md, docs/en/README.md, docs/ru/README.md, and docs/ko/README.md.
• All surrounding prose, link labels, other sponsors, and the existing sponsored/nofollow attributes are unchanged.
• All 20 existing AIXiamo shop URLs, their attributes, and UTM parameters are unchanged.
• No application, workflow, release, pricing, or business-condition changes.

The larger READMEs are deliberately not split or rewritten: this narrowly scoped repair changes only four existing href values.

Validation

• git diff --check passed.
• A byte-for-byte reversal check passed for all four files: restoring each original href reproduces the original file exactly.
• The preflight baseline is upstream main at 373a6f3.
• Application builds and runtime tests were not run because no application or build files changed.

I maintain the AIXiamo resource and am submitting this correction for the existing sponsorship entrance.